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Abstract

We present a Bessel beam illumination FDOCT setup using a FDML Swept Source at 1300nm with up to 440kHz A-scan rate, and discuss its advantages for structural and functional imaging of highly scattering samples. An extended focus is achieved due to the Bessel beam that preserves its lateral extend over a large depth range. Furthermore, Bessel beams exhibit a self-reconstruction property that allows imaging even behind obstacles such as hairs on skin. Decoupling the illumination from the Gaussian detection increases the global sensitivity and enables dark field imaging. Dark field imaging is useful to avoid strong reflexes from the sample surface that adversely affect the sensitivity due to the limited dynamic range of high speed 8bit acquisition cards. In addition the possibility of contrasting capillaries with high sensitivity is shown, using inter-B-scan speckle variance analysis. We demonstrate intrinsic advantages of the extended focus configuration, in particular the reduction of the phase decorrelation effect below vessels leading to improved axial vessel definition.

Theoretical lateral PSFs for the central wavelength in dB comparing the different configurations. The side lobes exhibit a stronger attenuation close to the focus in the extended tight focus, leading to a better resolution at that position, while the central lobe is less attenuated along the depth in the extended focus leading to a larger axial depth.

Measurement with a 4.5µm microsphere on a glass plate for several depths acquired with standard, extended tight focus and extended focus configuration in a, b and c respectively (numbers in µm; linear intensity scale). d: Average radial intensity profiles for selected depths showing similar FWHM at the focal position and the increase of the spot size for standard configuration with depth.

Effect of a hair on the signal attenuation along the depth. a, c & e: Standard, b, d & f: xf-configuration. a & b: Tomograms, c, d, e & f: En-face views at 2 different depths indicated by the white lines in the left tomogram showing a progressing reduction of the hair’s influence while it remains constant with the standard configuration. Scale bar denote 250µm in every picture.

Tomograms taken with the same glass plate orientation, almost perpendicular to the optical axis, with the reference power adjusted to avoid getting out of the range of the ADC. Left: Standard showing a limited penetration depth as well as a masked first interface, right: Extended focus remaining unaffected by the glass plate reflection. Scale bar denotes 250µm.

3D rendering of processed speckle variance stacks. 2x2mm section acquired with the extended focus setup. Left: Volume showing a large dynamic range of vessels for different depths (Media 1). Right: Volume taken at a different position showing a more planar vessel structure (Media 2).

a: 3D rendering of microcirculation embedded in the structural data. 2x2mm cut at different depths to show the planar microcirculation network close to the epidermis/dermis transition and bigger vessels deeper in tissue. b-d: En-face projection at a depth of about 330µm over 50µm (b), 460µm over 200µm (c), and 880µm over 120µm (d). e: Fusion of b, c & d microcirculation in the R, G & B color channels respectively. Scale bar denotes 250µm in every picture.